When does apparatus dead space matter for the pediatric patient?

Area of Science:

• Anesthesiology
• Pediatric Critical Care
• Respiratory Physiology

Background:

• Physiologic dead space is the lung volume without gas exchange.
• Apparatus dead space increases total dead space, potentially raising arterial CO2 (PaCO2) or minute ventilation requirements.
• Pediatric patients are particularly vulnerable to increased dead space due to their smaller tidal volumes.

Purpose of the Study:

• To quantify the impact of varying apparatus dead space on PaCO2 and minute ventilation in pediatric patients.
• To model the relationship between dead space volume (Vd) and PaCO2 in children up to 36 months old.
• To determine the respiratory rate (RR) needed to maintain normocapnia as dead space increases.

Main Methods:

• Calculated effects of dead space on PaCO2 and minute ventilation for infants and children (2-17 kg).
• Used manufacturer data and water displacement to determine apparatus dead space (Vd) for typical devices (8-55 mL).
• Derived relationships from the Bohr equation and Brody's equation for CO2 production (VCO2).

Main Results:

• PaCO2 increased exponentially with increasing apparatus dead space.
• Smaller patients experienced more rapid PaCO2 increases for a given change in Vd.
• Required RR to maintain PaCO2 at 40 mm·Hg also increased exponentially with dead space.

Conclusions:

• Increasing apparatus dead space can lead to exponential increases in PaCO2 and/or the required respiratory rate.
• The impact of dead space on PaCO2 is less significant in heavier children but remains considerable for typical circuit components up to 17 kg.
• Minimizing apparatus dead space is crucial for maintaining adequate ventilation in pediatric patients.